Chromium metal by reduction of chromic chloride with aluminum



Nov. 29, 1966 E. B. HOYT E TAL 3,288,596

CHROMIUM METAL BY REDUCTION OF CHROMIC CHLORIDE WITH ALUMINUM Filed Aug. 19, 1964 CrC1 A1 CHARGE- NQCl KCI A1C13 REACTOR AICI3 VAPOR FILTER $ALT5 Cr SPONGE ARGoN--- COOLER COLD WATER LEACHING WATER AND FILTER o|sso\ vED SALTS WET Cr- DRIER H2O DRY Cr PRODUCT INVENTORS: ERNEST B HOYT ALAN G.FOLL OWS AT ORNEY United States Patent O 3,288,596 CHROMIUM METAL BY REDUCTION OF CHRO- MIC CHLORIDE WITH ALUMINUM Ernest B. Hoyt, Geddes, and Alan G. Follows, Camillus, N.Y., assignors to Allied Chemical Corporation, New

York, N.Y., a corporation of New York Filed Aug. 19, 1964, Ser. No. 390,657 4 Claims. (Cl. 75-845) This invention relates to production of chromium metal and more particularly refers to a new and improved process for manufacture of chromium metal in the form of sponge or powder by reduction of chromic chloride with aluminum.

Chromium metal is finding increasing applications in the manufacture of stainless steel, special steel alloys and superalloys. Of particular interest is chromium in the form of sponge or powder which can be used both for alloying and in sintering or powder metallurgy forms. At present metallic chromium of the best commercial grade is made from the aqueous electrolysis of chromic sulfate solutions. This is believed to be a high-cost process. Chromium of poorer quality is made from chromic oxide by the aluminothermic and silicothermic processes, both of which are high cost. Other processes suggested by the art but not employed commercially because of inefficiency, high cost and impure product are reduction of CrCl with the alkali and alkaline earth metals, and two stage processes involving reduction of chromic chloride by hydrogen to chromous chloride followed by reduction of the chromous chloride with a metal.

An object of the present invention is to provide a method for the production of chromium metal by the reduction of chromic chloride with aluminum in a single step. Another object of the present invention is to concomitantly produce the valuable by-product anhydrous aluminum chloride, AlCl A further object of this invention is to produce a pure grade of chromium sponge or powder. Another object is to provide an efiicient economical process for converting chromium chloride into chromium sponge or powder. Other objects and advantages will be apparent from the following description and accompanying drawing.

In accordance with the present invention metallic chromium in the form of sponge or powder is produced by intimately mixing chromic chloride with aluminum in an amount of between 90-98% preferably 94-96% of the stoichio-metric amount required to convert the chromic chloride to chromium, adding to said mixture of chromic chloride and aluminum a carrier salt having a melting point below 900 C., to provide a molten medium in which the chromic chloride may dissolve, said carrier desirably being an alkali metal chloride, preferably a mixture of sodium chloride and potassium chloride, said carrier salt being in an amount from 15-65% preferably 45-55% of the chromic chloride, adding also to said mixture, anhydrous aluminum chloride in an amount of CLO-100% or more, preferably 4060% based on the chromic chloride, heating the mixture to a temperature of about 200-300 C. to effect initiation of the reaction with evolution of aluminum chloride vapor, releasing and recovering said aluminum chloride, continuing heating the reactants to a temperature within the range of 750950 C., preferably 875-925 C. and maintaining the reactants at the elevated temperature for a sufficient length of time to complete the reaction, separating the molten carrier salts from the resultant chromium sponge, cooling the sponge to a temperature below 100 C. preferably to about room temperature, i.e., 20-30 C., leaching out the residue of salt adhering to the cooled chromium sponge with cold water at a temperature within the range of to C. to produce high purity chromium sponge, and

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drying the sponge to remove adherent water, and pulverizin the sponge if desired to produce a powder. The carrier salts and by-product aluminum chloride are recycled.

Referring to the diagrammatic flow sheet, the primary reactants are chromic chloride and aluminum metal both in a finely-divided form to permit ready mixing to provide an intimate mix of the two reactants. The mix of aluminum metal to chromic chloride must be in the proper proportions. The quantity of aluminum admixed with the chromic chloride should be between 98% preferably 94-96% of the stoichiometric amount required to convert the chromic chloride to chromium metal. amount in excess of 98% of the equivalent aluminum results in undesirable residual aluminum left in the chromium metal and no practical method has been found to rid the residual metal of this aluminum. An amount of equivalent aluminum below 90% is inefiicient. To produce pure grade chromium sponge or powder the chromic chloride reactant should be as pure as possible and for this purpose there is available on the market resublimed chromic chloride containing 99.9% CrCl with insoluble of less than 0.05%. A pure aluminum powder or finely divided aluminum is also desirable.

The process of the resent invention also requires the inclusion of carrier salt with the chromic chloride and aluminum metal reactants so as to provide a molten salt medium to promote contact of the last small quantities of the reactants. Any anhydrous halides of the alkali or alkaline earth metals are suitable for this purpose. A mixture of equal proportions of NaCl and KCl, which gives a eutectic melting at 660 C. has been found very satisfactory, and is the preferred carrier medium but, if desired, either salt may be used alone. The amount of carrier salt used may vary from about 15% to 65% preferably about 45% to 55% based on the chromic chloride used. Higher amounts might be used without detriment to the chromium but would serve no useful purpose. The presence of such salts serves a very useful purpose in our process and are much more important when a reducing agent such as Al is used as compared to Na or Mg. These reducing agents give molten salts as products of the reaction but Al gives the volatile AlCl which sublimes out of the reaction zone immediately leaving a nearly solid powder.

The inclusion of aluminum chloride as part of the charge resulted in unexpected benefits. One effect of the incorporation of aluminum chloride into the charge was the reduction of reaction time by about 30 or more percent, i.e., a reaction which would normally take three hours at 900 C. would be completed in two hours due to the incorporation of aluminum chloride. The incorporation of aluminum chloride obviates the necessity of stirring the reactants. Stirring is a difficult operation, mechanically, owing to jamming of the agglomerated chromium between the stirrer and crucible walls. With the incorporation of aluminum chloride stirring is not required and will give a high purity chromium metal containing aluminum in an amount less than about 0.1%. Other advantages are (l) initiation of the reaction at a lower temperature-about 100than in the absence of AlCl and (2) a less explosive initial reaction spread out over a little longer period of time, thus making easier the condensation of all the released AlCl To obtain these desired effects the quantity of aluminum chloride incorporated in the charge should be substantial 20-l00% preferably 40-60%.

The charge as shown in the drawing, of chromic chloride, aluminum, sodium chloride, potassium chloride and aluminum chloride is introduced in a chromized or chrome plated steel reactor which is vacuum tight and filled with argon. The first indication of a reaction occurs at about 200 C. and is accompanied by a considerable temperature increase and the evolution of large quantities of AlCl vapor. Additional heat is supplied to the reactants until th-e temperature reaches at least 750 but not over 950 C. The temperature is held at the maximum point for a period of about two hours to permit completion of the reaction. The preferred holding temperature for our process is about 900 C. A minimum of 750 C., where the carrier salts have become molten, can be used.

The holding temperature should be maintained for a suflicient length of time to permit completion of the reaction and hence a Cr metal with little or no residual Thus, at 900 C. about two hours are sufficient, while at 750 C, several more hours are necessary. The evolved aluminum chloride vapor from the reactor is condensed and the condensed aluminum chloride may be resublimed to give anhydrous aluminum chloride of a high purity which is a valuable byproduct. A portion of the aluminum chloride may be returned to the reactor as a part of the charge for treatment of an additional batch of chromic chloride. The reaction products are then filtered to recover as much of the carrier salts for recycle as possible. This filtration may be done by gravity or with suction provided air and moisture are excluded. The molten salts containing some dissolved AlCl separated by filtration are desirably returned to the reactor for admixture with an additional batch of chromic chloride.

The filtered chromium sponge with adhering residue of salt is transferred to the cooler where it is then allowed to cool under argon. The chromium sponge cooled to a temperature below 100 C. preferably cooled to a temperature of about 2030 C. is pulverized and then leached by adding slowly to cold agitated water. The leaching step is important. Unexpectedly, it was found that chromium recovered could be increased greatly, or more, by leaching in cold (below 10 C.) or ice water. Dilute HNO (5%) is also a satisfactory leaching agent. The quality of the Cr is equal to that from the water leaching, but the Cr recoveries tend to be somewhat less for acid leaching and therefore water is the preferred leaching agent. After several leachings, the chromium sponge is filtered and the filtrate which is water containing dissolved salts is sent to waste. The chromium metal after filtrating contains some moisture and is dried at a temperature of about 100120 C. to produce a pure grade chromium metal product containing less than 0.1% aluminum with a chromium recovery of over 80% based on the aluminum used.

The following example illustrates the present invention.

The apparatus consists of a reactor constructed of a 4-inch diameter stainless steel pipe flanged at the top. This pipe is fitted with a stainless steel liner or crucible for holding the reactants and carrier salts. The liner has a heavily chromized inside surface. The flange cover is sealed with a garlock gasket and a gas inlet pipe, which can also accommodate a thermocouple well reaching into the reactants in the crucible, is sealed into the flange. A l-inch stainless steel tube serves as an outlet for AlCl vapor. This tube leads into a larger tube 4 x 8 inches which serves to condense the AlCl The argon gas, used to provide an inert atmosphere for the entire system, passes from the reactor to the condenser then to a trap and finally through H 804, which serves as a seal. The stainless steel tube, connecting the cover flange and condenser, is covered with heating tape to maintain a temperature of about 250 C. for the purpose of avoiding AlCl condensation and plug-up in the tube. A 220- volt electrical furnace, constructed of two semi-cylindrical pre-case heating elements, has a 5 x 9-inch deep chamber.

Two hundred grams of high purity resublimed CrCl together with 32.2 grams of Al dust, equal to 95% of the theoretical requirements, were pulverized and blended in a high-speed mixer, then 25 grams each of NaCl and KCl were blended into the mix. Finally 100 grams of AlCl were added and the whole mixed together. The reactor crucible was charged with the mix, the flange cover secured and the remaining equipment assembled. Argon was passed through the apparatus for an hour to purge it of air and moisture. On heating the reaction was initiated at 220 and the temperature rose to 490 in a short time. At the same time, a large volume of AlCl vapor escaped from the reactor and was collected in the condenser. The charge was heated to 900 and held for 2 hours without stirring. The reaction products are then filtered to separate the molten salts leaving a gray spongy mass containing an adhering residue of salt. The filtered chromium sponge is then allowed to cool under argon and the cooled sponge pulverized. The pulverized chromium product is leached with ice water until the leaching water gives a negative chloride test. The resultant product contained only 0.07% residual Al. The chromium recovery was 83.5% on the Al used.

For purposes of comparison 200 grams of chromic chloride were mixed with 32.3 grams of aluminum dust and heated. Activated reaction began at about 440 and the temperature was then raised to 900 and held at that temperature for 2 hours. The reaction product was leached with distilled water at room temperature (20 C.) and the resultant dry chromium sponge product produced was equivalent to a 70.5% recovery based on the Al used. The chromium sponge contained 2.6% residual aluminum.

Although certain preferred embodiments of the invention have been disclosed for purpose of illustration, it will be evident that various changes and modifications may be made therein without departing from the scope and spirit of the invention.

We claim:

1. A process for the production of chromium metal which comprises intimately mixing chromic chloride with aluminum in an amount by weight of between 98% of the stoichiometric amount required to convert the chromic chloride to chromium, adding to said mixture of chromic chloride and aluminum a carrier salt having a melting point below 900 C. to provide a molten medium in which the chromic chloride may dissolve, said carrier salt being in an amount from 15-65% of the chromic chloride, adding also to said mixture, anhydrous aluminum chloride in an amount of 20100% based on the chromic chloride, heating the mixture to a temperature of about 200-300 C. to eifect initiation of the reaction with evolution of aluminum chloride vapor, releasing and recovering said aluminum chloride, continuing heating the reactants to a temperature within the range of 750950 C. and maintaining the reactants at the elevated temperature for a sufficient length of time to complete the reaction, separating the molten carrier salt from the resultant chromium metal sponge, cooling the sponge to a temperature below C., and leaching out the residue of salt adhering to the cooled chromium metal to produce high purity chromium metal.

2. A process for the production of chromium metal which comprises intimately mixing chromic chloride with aluminum in an amount by weight of between 94-96% of the stoichiometric amount required to convert the chromic chloride to chromium, adding to said mixture of chromic chloride and aluminum a mixture of sodium chloride and potassium chloride as carrier salts to provide a molten medium in which the chromic chloride may dissolve, said carrier salt being in an amount from 45-55% of the chromic chloride, adding also to said mixture, anhydrous aluminum chloride in an amount of 40-60% based on the chromic chloride, heating the mixture to a temperature of about 200-300 C. to effect initiation of the reaction with evolution of aluminum chloride vapor, releasing and recovering said aluminum chloride, continuing heating the reactants to a temperature within the range of 875925 C. and maintaining the reactants at the elevated temperature for a suflicient length of time to complete the reaction, separating the molten carrier salt from the resultant chromium metal sponge, cooling the sponge to a temperature of about 2030 C., leaching out the residue of salt adhering to the cooled chromium sponge With cold water at a temperature within the range of 0 to 10 C. to produce high purity chromium metal.

3. A process as claimed in claim 1 in which at least 10 a part of the aluminum chloride employed as the charge is aluminum chloride from a previous operation.

4. A process as claimed in claim 1 in which at least a part of the carrier salt employed as the charge is a carrier salt from a previous operation.

References Cited by the Examiner UNITED STATES PATENTS 2,766,111 10/1956 Singleton 7584.5 2,837,420 6/1958 Doerner 75-845 2,913,333 11/1959 Eaton et al. 75-84.5 3,043,679 7/1962 Campbell et a1. 75-845 DAVID L. RECK, Primary Examiner.

N. F. MARKVA, Assistant Examiner. 

1. A PROCESS FOR THE PRODUCTION OF CHROMIUM METAL WHICH COMPRISES INTIMATELY IMXING CHROMIC CHLORIDE WITH ALUMINUM IN AN AMOUNT BY WEIGHT OF BETWEEN 90-98% OF THE STOICHIOMETRIC AMOUNT REQUIRED TO CONVERT THE CHROMIC CHLORIDE TO CHROMIUM, ADDING TO SAID MIXTURE OF CHROMIC CHLORIDE AND ALUMINUM A CARRIER SALT HAVING A MELTING POINT BELOW 900*C. TO PROVIDE A MOLTEN MEDIUM IN WHICH THE CHROMIC CHLORIDE MAY DISSOLVE, SAID CARRIER SALT BEING IN AN AMOUNT FROM 15-65% OF THE CHROMIC CHLORIDE, ADDING ALSO TO SAID MIXTURE, ANHYDROUS ALUMINUM CHLORIDE IN AN AMOUNT OF 20-100% BASED ON THE CHROMIC CHLORIDE, HEATING THE MIXTURE TO A TEMPERATURE OF ABOUT 200-300*C. TO EFFECT INITIATION OF THE REACTION WITH EVOLUTION OF ALUMINUM CHLORIDE VAPOR, RELEASING AND RECOVERING SAID ALUMINUM CHLORIDE, CONTAINUING HEATING THE REACTANTS TO TEMPERATURE WITHIN THE RANGE OF 750-950*C. AND MAINTAINING THE REACTANTS AT THE ELEVATED TEMPERATURE FOR A SUFFICIENT LENGTH OF TIME TO COMPLETE THE REACTION, SEPARATING THE MOLTEN CARRIER SALT FROM THE RESULTANT CHROMIUM METAL SPONGE, COOLING THE SPONGE TO A TEMPERATURE BELOW 100*C., AND LEACHING OUT THE RESIDUE OF SALT ADHERING TO THE COOLED CHROMIUM METAL TO PRODUCE HIGH PURITY CHROMIUM METAL. 